The present invention relates in general to ventricular support pumps and, controls, and, more specifically, to a ventricular assist device for reducing load applied to a weakened ventricle during the systolic phase.
Many types of circulatory assist devices are available for either short term or long term support for patients having cardiovascular disease. For example, a heart pump system known as a left ventricular assist device (LVAD) can provide long term patient support with an implantable pump associated with an externally-worn pump control unit and batteries. The LVAD improves circulation throughout the body by assisting the left side of the heart in pumping blood. One such system is the DuraHeart® LVAS system made by Terumo Heart, Inc., of Ann Arbor, Mich. The Duraheart® system employs a centrifugal pump with a magnetically levitated impeller to pump blood from the left ventricle to the aorta. The impeller can act as a rotor of an electric motor in which a rotating magnetic field from a multiphase stator couples with the impeller and is rotated at a speed appropriate to obtain the desired blood flow through the pump.
A typical cardiac assist system includes a pumping unit, drive electronics, microprocessor control unit, and an energy source such as rechargeable batteries and/or an AC power conditioning circuit. The system is implanted during a surgical procedure in which a centrifugal pump is placed in the patient's chest. An inflow conduit is pierced into the left ventricle to supply blood to the pump. One end of an outflow conduit is mechanically fitted to the pump outlet and the other end is surgically attached to the patient's aorta by anastomosis. A percutaneous cable connects to the pump, exits the patient through an incision, and connects to the external control unit. An LVAD system may be used with or without a pacemaker.
A control system for varying pump speed to achieve a target blood flow based on physiologic conditions is shown in U.S. Pat. No. 7,160,243, issued Jan. 9, 2007, which is incorporated herein by reference in its entirety. A target blood flow rate may be established based on the patient's heart rate so that the physiologic demand is met. In one type of conventional control unit, a constant speed setpoint has been determined for the pump motor to achieve the target flow based on demand. In this type of system, the pump speed is substantially constant within an individual cardiac cycle.
Pulsatile pumping systems are also known wherein the pump speed is varied within the cardiac cycle to more closely mimic natural heart action. In one example, U.S. Pat. No. 8,096,935 to Sutton et al oscillates the speed of the pump to produce a pulsed pressure. The speed is oscillated synchronously with the natural cardiac cycle such that a pump speed is increased during systole (the time of highest flow) and decreased during diastole (the time of lowest flow).
Whether operated at a constant speed or in a pulsatile manner, it is known that when desiring to obtain a maximum unloading of a weakened ventricle the average pump speed should be increased as much as possible (so that the pump flow is increased to the point where it captures almost the entire cardiac output). Due to flow inertia, however, the pump flow lags the ventricular pressure increase occurring at the beginning of systole. Therefore, the ventricle contraction still remains isometric at the beginning of systole (i.e., the pressure inside the ventricle resists its contraction). Furthermore, an increased average pump speed increases the risk of ventricular suction, particularly at the end of systole when the ventricle could be nearly empty.